Hypersonic flight has been an interesting area of research for the past half-century, owing to the complex aerodynamics, high temperature, and stresses associated with it. Hypersonic flight involves materials traveling at a Mach number greater than 5.5; simply, more than 5.5 times the speed of sound.
At such high speeds, massive thermal and aerodynamic stresses are being applied to the airframe and other structural parts of the aircraft. This necessitates the use of materials with exceptional tensile and thermal attributes, along with functionalized special coatings and specific design methodology, to ensure structural strength and durability.
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Thermal Issues with Hypersonic Flight
The technique of heat flux into the wall is traditionally utilized for determining the thickness of the reusable Thermal Protection System (TPS) for hypersonic vehicles. Typically, the TPS thickness is not uniform; it tends to decrease in the downstream direction of the flow past the TPS. The cooling of the TPS is achieved through passive sources, primarily thermal surface radiation.
The significant challenge in scientific and air-vehicle engineering is the difficulty in accurately and reliably predicting the location and extent of the laminar-turbulent transition zone. This issue has been highlighted by researchers from the United States in an article published in the CEAS Space Journal. This transition significantly affects thermal loads (and consequently, structural weight), viscous drag, and the onset flow in engine inlets.
The recorded temperatures at the vehicle surface for hypersonic flight have been around 2500°C. This value is significantly higher than the melting temperatures of strong metals like steel and titanium. However, effectively handling external heat on a hypersonic weapon is just one challenge, and it is further complicated by the presence of heat-generating electronics inside.
Materials and Design Considerations for Hypersonic Flight
Material requirements for hypersonic flight are associated with the vehicle's design and flight envelope, presenting two primary environmental challenges. These factors are mentioned in a research article published in ArXiv. The first involves thermal loads, influenced by both the geometry and location of the vehicle. The second challenge is posed by highly oxidizing conditions, leading to alterations in both material properties (oxidation) and geometry (ablation).
Material requirements become more challenging for hypersonic flight due to the dissociation of O2 and N2 into free radicals at gas-phase temperatures exceeding 3000°C. These extreme conditions result in highly reactive surface chemical interactions, leading to material degradation, microstructural changes, phase formation, and property alterations during flight.
Materials designers face critical challenges, especially in the design of leading-edge surfaces exposed directly to aerothermal conditions and in the propulsion flow path where radiative cooling is not feasible.
The preliminary materials selection is efficiently done through thermo-mechanical simulations. The simulations involve specific material properties and conditions (such as heat flux and stagnation temperature) being imposed on a component to compute the resulting thermal profile.
This profile is then utilized as boundary conditions to calculate thermal stresses. This screening proves valuable in determining whether the maximum temperature on a surface during hypersonic flight exceeds the melting point of the materials and/or if the thermal stress values are much greater than the material's flow stress at the given temperature.
New Materials are Being Developed for the Thermodynamic Loads during Hypersonic Flight
Research is being conducted all over the world to develop specialized materials to be used on vehicles intended for hypersonic flights. The University of Southern Queensland (USQ) is actively researching the development of novel materials for hypersonic flight. The research team is focused on materials with high-temperature resistance and extended durability to advance reusable launch vehicles and engines.
In collaboration with the private company Hypersonix, they are working on the development of a groundbreaking DART CMP drone with specialized thermal materials for hypersonic flight.
At its core, the drone will feature a 3D-printed Spartan hydrogen-fueled scramjet engine. The team at Hypersonix is involved in the assembly phase of its DART AE 3D-printing demonstrator craft, and after this, the focus will shift to the DART CMP, emphasizing the integration of composite materials. The materials will be able to withstand high temperatures not only from the hypersonic flight but also from the propulsion system.
The utilization of 3D printing not only leads to cost savings but also results in a more resilient and seamlessly assembled product, with the added benefit of a swift three-week printing timeframe for each new unit.
Why is Silicon Nitride a Good Choice for Hypersonic Flight?
Silicon nitride is a lightweight but durable aerospace material and is being used in vehicles flying at hypersonic speeds. Silicon Nitride can withstand extremely high temperatures; that is the core reason it is being used in structural parts of hypersonic flight vehicles.
Dense silicon nitride is an exceptionally tough, abrasion-resistant, and corrosion-resistant solid material. In comparison to more familiar ceramics like porcelain or glass, silicon nitride exhibits remarkable strength, boasting the highest fracture resistance among advanced ceramics. This exceptional quality allows it to endure challenging operational conditions that might otherwise lead to deformation or fracture in other ceramic materials.
What are New Ultra High-Temperature Ceramics for Hypersonic Flight?
A novel category of hybrid materials, referred to as ultra-high-temperature ceramic matrix composites (UHTCMCs), can endure high thermal shocks and withstand critical mechanical stresses.
This innovative class combines lightweight ceramic matrix composites known for their high thermal shock resistance and toughness. Furthermore, these ultra-high-temperature ceramics also exhibit lower erosion beyond a specific temperature.
These hybrid materials offer flexibility for experimental adjustments and customization of their structure across various scales. Notably, their uniqueness lies in the utilization of advanced processes that streamline synthesis time and reduce costs.
Under specific conditions, UHTCMCs demonstrate the ability to repair initial damage before it spreads. When subjected to thermal stress, the incorporation of nano-sized substances in the ceramic material prompts the formation of an external solid protective layer and an internal liquid phase, facilitating the healing of flaws. This self-healing characteristic enhances the reusability of rockets for multiple re-entries.
Hypersonic flight is demanding; it can lead to severely high temperatures and thermal stresses, which necessitate the use of materials with exceptional qualities. High-strength materials which can be used for the airframe as well as for the airframe structures are being researched extensively. With each passing day, researchers are developing new materials fabricated via modern technology to continuously improve performance during hypersonic flight.
Determining Microstructural Characteristics of Metals for Aerospace
References and Further Reading
Cordis-European Commission, (2023). Hybrid Ceramic Materials that take the heat could enable hypersonic flight. [Online]
Available at: https://cordis.europa.eu/article/id/429161-hybrid-ceramic-materials-that-take-the-heat-could-enable-hypersonic-flight
Hirschel, H., Weiland, C. (2011). Design of hypersonic flight vehicles: some lessons from the past and future challenges. CEAS Space J 1, 3–22. Available at: https://doi.org/10.1007/s12567-010-0004-4
Pollock Group, (2023). Materials for Hypersonic Flight. [Online]
Available at: https://labs.materials.ucsb.edu/pollock/tresa/research/materials-hypersonic-flight
Reim, G., (2023). 7 technical challenges that need to be overcome by hypersonic missile builders. [Online]
Available at: https://www.flightglobal.com/flight-international/7-technical-challenges-that-need-to-be-overcome-by-hypersonic-missile-builders/138237.article
Peters B. et. al. (2023). Materials Design for Hypersonics. arXiv preprint arXiv:2309.04053. Available at: https://doi.org/10.48550/arXiv.2309.04053
Sharping, C., (2022). Silicon Nitride: A High Temperature Material For Hypersonic Technology. [Online]
Available at: https://sintx.com/hypersonic-technology/
The University of Queensland, (2022). The Biggest Challenges to Hypersonic Flight – and How they’re being tackled. [Online]
Available at: https://mechmining.uq.edu.au/article/2022/10/biggest-challenges-hypersonic-flight-%E2%80%93-and-how-they%E2%80%99re-being-tackled
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